DESCRIPTION OF THE PRIOR ART
Electron beam lithography systems provide a fundamental method of pattern generation for large scale integrated circuits (LSI) and very large scale integrated circuits (VLSI). These systems are very useful for making photolithographic masks for optical exposure, as well as direct writing on semiconductor wafers (i.e., defining patterns in resist with an electron beam).
This invention relates to a high precision, submicrometer positioning system, and is concerned particularly, though not exclusively, with a mechanical system for precisely manipulating workpieces within an electron beam lithography system.
A basic electron beam lithography system may include an electron beam source, condenser lenses, de-magnification lens stages, a projection lens, a deflection unit, and a target area (see U.S. Pat. No. 3,644,700 issued Feb. 22, 1972 to Kruppa et al). As demands for high throughput and denser circuitry increase, electron beam systems are becoming more complex. For example, movable positioning tables and high precision positioning means such as laser-interferometer positioning sub-systems have been incorporated into electron beam exposure systems.
To achieve the relatively high throughputs required of a manufacturing lithography system, efforts have previously been made to increase the accuracy, and reduce the time involved when transporting workpieces (i.e., wafers, photolithographic plates, or masks), to and from a workpiece holder or fixture which ultimately manipulates the masks, plates or wafers as the electron beam operates on them. U.S. Pat. Nos. 3,874,525 and 3,968,885, both by Hassan et al and assigned to this assignee deal with such a transfer mechanism.
As circuit densities further increase and wafers increase in size (i.e., 82 mm and 25 mm), the problem of producing masks with low mask to mask overlay specifications, while achieving manufacturing level throughputs becomes more significant. To create present day semiconductor devices, approximately 7 to 20 mask overlays are required to form a wafer. If mask to mask overlays are large, then vias and other features may be misaligned, and the specifications required for the dense packaging of devices could not be repeatedly achieved. As wafer and mask sizes increase, the adverse affect on yields increases as mask to mask overlay error increases.
A concise, detailed description of a state of the art electron beam system, of which the problems and background described herein would apply is described in "EL Systems: Throughput Electron Beam Lithography Tools" by R. D. Moore, Solid State Technology, September 1983, pp. 127-132. The aggressive performance specifications for the EL-3 system are described in the Moore article, see Tables IV and V, (for example, the mask maker specification calls for throughputs of up to 10 82 mm masks per hour at a current density of 25 amps/cm.sup.2, and 0.35 micrometer mask to mask overlay for 1.times. mode masks). The present invention's major application is to provide the mechanical precision and repeatability to make performance specifications such as those listed in the Moore article possible.
Complex electron beam mask maker systems, such as the system described above, inherently do not produce masks which have no overlay error. The system's mask to mask overlay capability is mainly a function of the accuracy of the electron beam column, electronics associated with the workpiece positioning apparatus (i.e. feedback from interferometer), and mechanical hysteresis (i.e. non-repeatable distortion) in the system. In state of the art systems, mechanical hysteresis accounts for a significant portion of the system's mask to mask overlay differential. In order to have tolerable and repeatable mask to mask overlay capability, mechanical hysteresis must be reduced in a manner that will not adversely effect the electron beam performance (i.e., metals in vicinity of electron beam cause beam deflection). The electron beam pattern generating equipment must be capable of submicrometer repeatability and precision.
Common sources of mechanical hysteresis in a state of the art, high throughput, electron beam mask maker system include friction interfaces, which are found in non-integral bodies or systems comprised of numerous components joined by transient means such as bolts or screws at component interfaces. Friction interfaces decrease system stability due to potential non-repeatable slip at such interfaces. Thermal stresses due to the different thermal expansion coefficients of various sysstem components and forces transmitted due to the movement and imperfections of the movable stage also contribute to mechanical hysteresis.
Friction interfaces, stresses and forces which are transmitted through the system, all affect the stability of the assembled body or structure. This affect becomes more significant for systems and equipment such as the electron beam systems described above, which must be capable of submicrometer precision and repeatability.
From a stability and precision point of view, an integral elastic body is ideal because after being stressed, an integral elastic body will return to its origin when relaxed, thus exhibiting no mechanical hysteresis. Practically, a complex electron beam system, or any complex precision system cannot be made of one integral body. The structures, such as the mechanical positioning structure of the present invention, are assembled with many parts. Thus, emulating the property of an elastic body would minimize instability in a multi-component precision positioning system, such as is required for electron beam system applications.
The versatility of a precision positioning system, (i.e., the system which is described herein contains a mask/wafer coupling means which can handle various size workpieces, and can easily be engaged or disengaged from the supporting platform), may also contribute to mechanical hysteresis, since additional friction interfaces are added for the easy engagement/disengagement of workpieces or carriers.
Mirror surfaces that move with the workpiece are commonly used in a laser interferometer positioning system. In such systems, the distance between the mirror surfaces and the workpiece is critical. If the distance between the workpiece and mirror surface is not substantially fixed (i.e. if movement of the plate occurs once plate writing starts), then the address that the movable stage is sent by the laser interferometer system will not correspond to the workpiece area that the electron beam is supported to write on. If this distance is not substantially fixed, the mask to mask overlay will be adversely affected. Mechanical hysteresis must be minimal so that the distance between the interferometer mirror surface and workpiece can be as close to a fixed distance as possible.
To eliminate mechanical hysteresis, it is desirable to have a stable mechanical system, with as few interfaces as possible, and integral construction made of materials which exhibit elastic properties.
Mechanical stability implies an integral construction, but no single material is completely suitable for the inherent requirements of a high-current electron beam system. The material must not be ferromagnetic, or even paramagnetic or diamagnetic; otherwise the beam can be displaced by the interaction of the material near the high fields of the electron beam lens. The material must be an insulator, or charge accumulation on exposed surfaces could deflect the beam. The material must not be a good bulk conductor, or the magnetic fields generated by eddy currents could deflect the beam.
Failure to meet any of these requirements would result in unacceptable electron beam distortion. These requirements are especially important for the present manufacturing level throughput mask making system, in which high electron beam currents are required. Since metals cannot be used in the proximity of the electron beam, the problem of reducing mechanical hysteresis in the mechanical positioning system is compounded. Alternative materials, such as glasses and plastics, have their own problems (even if plated to drain surface charge). Glass is brittle and fragile. Plastic is relatively unstable; it may flow and relax when stressed, rather than responding elastically.
In view of the compounded problem of reducing mechanical hysteresis due to the material constraints involved in an electron beam system, the problem of maintaining a substantially fixed distance between the interferometer mirrors and workpiece is also compounded. It is of further significance, that the electron beam mask making system described herein has no registration marks as direct wafer write systems do. The main reasons for this are that it is cost effective to not use valuable mask real estate for registration marks, and that throughput is increased if the electron beam can continually write, without consuming time for re-registration steps. Since the mask making electron beam system uses no registration marks, an entire photolithographic plate must be "blind" written, which causes any positioning errors and workpiece movement that are compounded by drift in the fixed distance between the workpiece and the laser interferometer mirror surfaces.
As noted above, a movable stage is used for ultimately positioning the workpiece with respect to the electron beam. Both the acceleration of the movable table (i.e., gravitational acceleration rates and higher are utilized to attain the speeds required for specified high level throughputs), and stage imperfections contribute to mechanical hysteresis in the system. Acceleration and deceleration also contribute to the problem of "ringing", which is the problem which occurs after the stage stops, and it continues to vibrate for a while, during which time the exposure operation must be suspended. U.S. Pat. No. 4,117,240 by Goto et al describes an electron beam exposure system in which the problem of vibration after the stage stops is highlighted. However, the Goto et al patent does not deal with the problem from the point of view of reducing overall mechanical hysteresis in the system. To maintain high level throughputs, the system cannot wait for "ringing" to cease before it starts to write on the photolithographic plate. Furthermore, in accordance with the mechanical constraints and problems described above, all clamping arrangements must not transmit any significant distorting forces to the workpiece (i.e. photolithographic plate) or workpiece supporting superstructure. Conventional clamping means that clamp a flat workpiece (i.e. plate), typically do so with forces applied across the entire length of the workpiece, which introduces distorting forces.
General problems associated with submicrometer dynamic positioning repeatability in electron beam systems and the like have been addressed previously. However, none of the prior art deals with electron beam specifications as aggressive as those of the aforementioned EL-3 system. U.S. Pat. No. 4,103,168 by Sturrock et al concerns reducing unwanted movement between the mirrors and interferometer heads of an electron beam microfabrication apparatus which incorporates an interferometer positioning system. That patent is particularly concerned with reducing unwanted movement between the mirrors and interferometer heads. However, the effect of mechanical hysteresis on the physical relationship between the interferometer mirrors and workpiece (i.e. substrate) is not shown or described. Retaining a substantially fixed distance between the mirrors and workpiece is vital to the present invention. U.S. Pat. No. 3,934,187 by Trotel concerns the interferometer positioning of a substrate. The need for the substrate to be bombarded in a perfectly defined position in relation to the X and Y axes of the interferometer mirrors is recognized, but not shown or described.
U.S. Pat. No. 3,648,048 by Cahan et al describes a system for positioning a wafer and controlling the displacement of the wafer in an electron beam apparatus. The system uses interferometers and piezoelectric actuators to position wafers. The workpiece is adjusted via alignment marks. As stated above, the present invention allows for precise submicrometer repeatability without the use of registration or alignment marks. Many other prior art systems, such as that disclosed in U.S. Pat. No. 4,370,554 rely on registration or alignment marks.
U.S. Pat. No. 3,521,056 by Suzuki concerns a means to correct for thermal stresses in an electron beam system. However, this patent does not concern the problem of mechanical hysteresis.
"A position-controlled rectangular-coordinate table" by Garside and Pickard in the JOURNAL OF PHYSICS, E: Scientific Instrumentation, Vol. 16, March 1983, pp. 223-226 describes a precision positioning system for use with an electron beam apparatus. The system uses laser interferometer measuring to determine the position of an X-Y stage. A work carrier is mounted on the X-Y stage by balls made of a non-ferromagnetic material (sapphire). Workpiece holders are mounted on the work carrier by sapphire balls also. The interferometer mirrors are mounted to the X-Y stage.
The Garside system is not capable of meeting the overall submicrometer precision and throughput levels that the present invention was created to meet. Numerous frictional interfaces, including the sapphire balls, add to the overall mechanical hysteresis of the system. In addition, the interferometer mirror is not in the same plane as the workpiece. Thus, any error at the interferometer mirror is amplified. In sum, Garside provides for submicrometer positioning of an X-Y stage only, and doesn't recognize the problems of overall system precision and repeatability in view of mechanical hysteresis.
In view of the above, it is evident that a need exists for a high precision, submicrometer positioning system for electron beam lithography applications; that provides for high throughputs, low mask to mask overlay errors, low mechanical hysteresis, submicrometer dynamic position repeatability, and which also can continuously position a workpiece without registration marks (i.e. provide for "blind" write), provide for variable sized workpieces, withstand high acceleration forces, and provide non-distorting workpiece clamping.